Archive for the ‘Oakland geology puzzles’ Category

At the back of Mills College is a steep little hill called Pine Top. It looms especially high over Lake Aliso. The geologic map shows it as consisting of Jurassic basalt.

As I walked up Pine Top Road to reach it, I saw only what looked like nondescript sandstone, but I didn’t look closely. Rocks may be altered, and the Hayward fault running through here is notorious for swapping splinters of rock from one side to the other, so who knows.

As you round the hill, the view opens across Seminary Avenue, which runs up the valley of Chimes Creek, to Millsmont hill.

At the top is this big old stone stage, where it’s easy to imagine the young women of Mills assembling for all kinds of ceremonial occasions.

What’s harder to imagine is the view that this place once commanded. All the trees are so mature, both pines and eucalyptus, that it’s frustrating.

On the other hand, people live on all sides now, so privacy is more important. And the freeway behind the hill is so noisy that any ceremony you could think of would be spoiled by the din.

As I said, there was no obvious evidence of basalt exposed along the road to Pine Top. So what was this lump of conglomerate doing there behind the stage?

Just another reason to come back and poke around the flanks of this hill.

Longtime commenter artisancrafts reminded me, in a comment to my last post, that there’s a nice exposure of blueschist at the north edge of Mills College. Yesterday I easily located it by following his/her directions. First there’s the old railbed that once ran to Laundry Canyon. This stretch of it, which once continued down through the Fan parallel to High Street, used to be part of Courtland Street.

At the spot in the distance where the sun shines across the roadbed is this lovely exposure, about 2 by 3 meters.

It’s clearly worn by water, and a little concrete-lined ditch running along the uphill side of the roadbed feeds it. Following it upstream takes you right up to the 580 freeway abutment, where it veers north in a culvert. The stream is a mystery that I won’t try to solve today.

Back to the rock. Getting closer to it, you may not believe your camera. Clear blue skylight can do that.

This closeup, showing tightly folded lamination in the cleft on the right edge of the first shot, is a truer indication of its color thanks to my camera’s flash.

It’s classic blueschist, the largest outcrop of it I’ve seen in Oakland. Let’s call it blueschist-grade melange, what’s usually referred to in Franciscan circles as a high-grade block, and I’m very pleased to know that we have one in town.

Geranium Place occupies a sloping bit of land just north of Horseshoe Creek below Redwood Road. This map shows the location plus the sites of the photos in this post.

The bedrock here is mapped as Franciscan melange on the west and the Leona volcanics on the east, the same stuff exposed in the huge Leona Quarry just down the Warren Freeway. The rocks I saw were clearly the latter, but you have to take the lines on the geologic map as approximations. If I’d been there fifty years ago when the homes here were being built I might have been able to tell better, while the foundations were exposed. BUT! there is bedrock to look at in any case.

The first thing that caught my eye, though, was the gutters. What’s going on here?

Iron-rich runoff like this is not expected from undisturbed land. I think the area was once a borrow pit or small quarry. Across Horseshoe Creek is the big quarrying operation above Laundry Canyon, and just beyond is the notorious McDonell sulfur mine site, so naturally the resemblance to the nasty-looking streambed down there is striking. What etched away the cement in the gutter? My best guess is acid drainage.

The white crust is another clue. In our deathly dry conditions it might conceivably have been salt, but it had no taste when I nibbled a fragment of it. It’s probably either gypsum or carbonate; without any chemicals handy I couldn’t learn anything more. But a buildup of crystals like this could gradually disintegrate the cement in the gutter. It may also be seepage from the serpentine exposed along and above Redwood Road here. In sum, very hard water here, but probably not nasty water.

The bedrock is heavily iron-stained and chewed up. Being so near the Hayward fault (it grazes the lower left corner of the map area) surely accounts for that.

Some of the households here have worked with it to good effect.

A closeup is impressive.

This is breccia—pervasively shattered rock—that has been abraded by tectonic shearing so the pieces are rounded, as if you took crushed rock and rubbed it between your hands with a giant’s strength. It’s fairly well cemented, not crumbling apart, so this process happened at some depth.

At the northernmost bend of Geranium, up against the highly cantilevered Redwood Road, the ground is empty and there are monitoring wells of some sort. We have deeply disturbed this area.